This invention relates to a coherent oscillator which samples an input signal so that it can later regenerate the input signal's phase and frequency. More specifically, the present invention relates to demodulating reflected radar pulses utilizing a coherent oscillator. Furthermore, the present invention relates to coherent-on-receive radars which may find such a coherent oscillator particularly useful.
A radar transmits a short pulse of radio frequency energy and then receives a reflected portion of the energy during a pulse repetition interval. The reflected energy must be demodulated before information contained therein can be processed. Effective demodulation of the reflected energy often requires an oscillation signal which exhibits phase and frequency characteristics corresponding to those exhibited by the transmit pulse. However, at a time when such a signal is needed, the transmitter portion of a radar has ceased transmitting the transmit pulse and the transmit pulse is not available to aid generation of such a signal. Furthermore, a coherent-on-receive radar employs a noncoherent transmitter, such as a magnetron, which exhibits a random phase relationship from transmit pulse to succeeding transmit pulses. Thus, coherent-on-receive radars need a coherent oscillator which samples a transmit pulse as it is being transmitted and regenerates the phase and frequency of the transmit pulse as reflected energy is being received.
Coherent oscillators and demodulation techniques for coherent-on-receive radars are known in the prior art. One coherent oscillator technique utilizes a relatively conventional phase locked loop circuit which locks onto a phase and frequency representative of a transmit pulse and then holds the phase and frequency during the pulse repetition interval. However, a loop filter portion of a phase locked loop which might permit locking within the time interval of a transmit pulse would not permit holding of the sampled signal for an adequately long pulse repetition interval. Conversely, a loop filter which permits the phase locked loop to remain in lock during the pulse repetition interval does not permit the phase locked loop to achieve a locked condition within a relatively short time period of the transmit pulse. Thus, this phase locked loop coherent oscillator requires the radar to utilize undesirably long transmit pulse intervals and undesirably short pulse repetition intervals.
One relatively unconventional prior art phase locked loop circuit permits an oscillator portion of the phase locked loop to exhibit a phase and frequency characteristic of the inlock condition longer than would otherwise be permitted by the loop filter. U.S. Pat. No. 4,101,844 issued to Hugh Robert Malone describes this phase locked loop. The Malone circuit achieves a locked condition in a closed loop mode while a signal is being received. In this closed loop mode a sample and hold amplifier acquires and outputs a voltage level representative of a signal which controls operation of a VCO portion of the circuit. Then, the received signal disappears and the circuit switches to an open loop mode where the VCO is controlled by the voltage level acquired and output by the sample and hold amplifier. While this phase locked loop circuit is suitable for its intended purpose as a system oscillator for transponders, it is inoperative in radar applications where short transmit pulse intervals do not permit the locking of a phase locked loop.
Additionally, digital techniques have been employed in the demodulation of coherent-on-receive radar signals, as described in U.S. Pat. No. 4,095,224 issued to Eric A. Dounce and Guy V. Morris. Although these digital techniques are effective in a variety of radar applications, they are undesirably complex, costly and troublesome.